Sebastian Kuzminsky wrote:
> Sounds good. Your encoder has 512 lines, so it produces 2048 pulses per
> revolution. Your machine is metric, so INPUT_SCALE is counts/mm, which
> is 2048 counts/rev / 2.5 mm/rev = 819.2 counts/mm.
Yes, luckily I was able to verify this with my logic analyzer - the
company selling the units specified 500 lines ...
>> Somehow I can not get my motors working from this point on. Anything I
>> do results in a "joint following" error. As soon as I power my motor the
>> slightest push on the motor results in the error.
>>
>
> It's expected that with the machine *out* of E-stop, manually turning
> the leadscrew should cause a following error. After all, emc2 now
> thinks it's controlling the machine, and it made an un-commanded move.
> So that's good too.
>
Anything includes doing a jog move.
With INPUT_SCALE = 81920 everything worked. The motors moved as
commanded even with jog speeds of 18 mm/min (which is 720 rpm with the
incorrect 0.025 mm/rev). But as soon as I set the INPUT_SCALE to 819.2
(2.5 mm/rev) I can't get it to work. I thought that setting the
OUTPUT_SCALE to 100 should do the trick since raw = output /
OUTPUT_SCALE - at least that is how I read it from section 7.2.9.2 in
the integrators manual.
How I got this:
with an input_scale of 81920 one rev = 0.025 mm and a certain output
results in 720 rpm being 18mm/min.
The new input_scale setting of 819.2 results in 2.5 mm/rev so the
same voltage to the motors now results in 1800mm/min.
Since voltage = (output - offset)/scale, the scale should be 100
times larger in order to create the same voltage for an output that
is 100 times more.
But it looks like the output_scale and input_scale are part of the whole
feedback loop since increasing the output_scale reduces the force that
is needed to push the motor out of the commanded position.
Just in case I was wrong: I tried both output_scale = 100 and 0.01
(original was 1.0), both result in a non-working system.
>
> Now you're in the tricky world of servo tuning... This is something i
> dont know much about, but i would start here:
>
> <http://linuxcnc.org/docs/devel/EMC2_Integrator_Manual.pdf>
>
> Specifically, section 2.2 talks about servos and servo tuning.
>
2.2 explains the basic concept. Section 29 goes a bit deeper into tuning
of the PID constants but OUTPUT_SCALE and MAX_OUTPUT are not mentioned
over there.
If the PID tuning is done for a given system I should be able to change
the unit dimensions without touching the PID parameters: PID parameters
are dependent on the electronic and the mechanical properties, not the
unit size.
Rob
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